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©2000 


Technical  and  Bibliographic  Notes  /  Notes  techniques  et  bibliographiques 


The  Institute  has  attempted  to  obtain  the  best  original 
copy  available  tor  filming.  Features  of  this  copy  which 
may  be  oibliographically  unique,  which  may  alter  any  of 
the  images  in  the  reproduction,  or  which  may 
significantly  change  the  usual  method  of  filming  are 
checked  below. 


Z 


D 
D 
D 


D 


Coloured  covers  / 
Couverture  de  couleur 


r~7]   Covers  damaged  / 


Couverture  endommag^e 


□    Covers  restored  and/or  laminated  / 
Couverture  restaur^e  et/ou  pellicul^e 

Cover  title  missing  /  Le  titre  de  couverture  manque 

I I   Coloured  maps  /  Cartes  g6ographiques  en  couleur 

□   Coloured  ink  (i.e.  other  than  blue  or  black)  / 
Encre  de  couleur  (i.e.  autre  que  bleue  ou  noire) 

□    Coloured  plates  and/or  illustrations  / 
Planches  et/ou  illustrations  en  couleur 

Bound  with  other  material  / 
Reli6  avec  d'autres  documents 

Only  edition  available  / 
Seule  Edition  disponible 

Tight  binding  may  cause  shadows  or  distortion  along 
interior  margin  /  La  reliure  serree  peut  causer  de 
I'ombre  ou  de  la  distorsion  le  long  de  la  marge 
int6rieure. 

Blank  leaves  added  during  restorations  may  appear 
within  the  text.  Whenever  possible,  these  have  been 
omitted  from  filming  /  II  se  peut  que  certaines  pages 
blanches  ajout^es  lors  d'une  restauration 
apparaissent  dans  le  texte,  mais,  lorsque  cela  ^tait 
possible,  ces  pages  n'ont  pas  6t6  film^es. 


L'lnstitut  a  microfilme  le  meilleur  exemplaire  qu'il  lui  a 
ete  possible  de  se  procurer.  Les  details  de  cet  exem- 
plaire qui  sont  peut-etre  uniques  du  point  de  vue  bibh- 
ographique.  qui  peuvent  modifier  une  image  reproduite. 
ou  qui  peuvent  exiger  une  modification  dans  la  metho- 
de  normale  de  filmage  sont  mdiqu^s  ci-dessous. 

Coloured  pages  /  Pages  de  couleur 

\l/\    Pages  damaged  /  Pages  endommagees 


D 


D 


Pages  restored  and/or  laminated  / 
Pages  restaur^es  et/ou  pelliculees 


yl    Pages  discoloured,  stained  or  foxed  / 
k_l    Pages  decolor^es.  tachetees  uu  piqu6es 

Pages  detached  /  Pages  d^tach^es 

1^    Showthrough  /  Transparence 

I      I   Quality  of  prim  varies  / 


D 
D 


Quality  in^gale  de  I'impression 

Includes  supplementary  material  / 
Comprend  du  materiel  suppl^mentaire 

Pages  wholly  or  partially  obscured  by  errata  slips, 
tissues,  etc.,  have  been  refilmed  to  ensure  the  best 
possible  image  /  Les  pages  totalement  ou 
partiellement  obscurcies  par  un  feuillet  d'errata,  une 
peiure,  etc.,  ont  ^t^  film^es  d  nouveau  de  fa?on  a 
obteni.-  la  meilleure  image  possible. 

Opposing  pages  with  varying  colouration  or 
discolourations  are  filmed  twice  to  ensure  the  best 
possible  image  /  Les  pages  s'opposant  ayant  des 
colorations  variables  ou  des  decolorations  sont 
film^es  deux  fois  afin  d'obtenir  la  meilleure  image 
possible. 


Additional  comments  / 
Commentaires  suppl6mentaires: 


Pagination  is  as  follows:  p.  373-379. 

La  pagination  est  comme  suit:  p.  373-379. 


This  item  is  filmed  at  the  reduction  ratio  checked  below  / 

Ce  document  est  filmi  au  taux  de  reduction  indirut  ci*dc«tou>. 


lOx 

14x 

IBx 

22x 

26x 

30x 

V^''^ 

12x 


16x 


20x 


24x 


28x 


32x 


The  copy  filmed  h«r«  hu  b««n  raproduead  thanks 
to  tha  ganarotity  of: 

Gerstein  Science  Information  Centre 
University  of  Toronto 

Tha  imagas  appaaring  hara  ara  tha  bast  quality 
poaaibia  considaring  tha  condition  and  lagibility 
of  tha  original  copy  and  in  kaaping  with  tha 
filming  contract  spiaeificationa. 


L'axamplaira  film*  fut  raproduit  grica  k  la 
g*n*rositi  da: 

Gerstein  Science  Information  Centre 
University  of  Toronto 

Las  imagas  suivantas  ont  M  raproduitas  avec  Is 
plus  grand  soin,  compts  tanu  da  la  condition  st 
da  la  nattst*  da  l'axamplaira  film*,  at  an 
conformit*  avac  laa  conditions  du  contrst  da 
filmaga. 


Original  copias  in  printad  papar  eovars  ara  fllmad 
baginning  with  tha  front  covar  and  anding  on 
tha  laat  paga  with  a  printad  or  illuatratad  impraa- 
sion,  or  tha  back  covar  whan  appropriata.  All 
othar  original  copiaa  ara  filmad  baginning  on  tha 
first  paga  with  a  printad  or  illuatratad  impras- 
sion,  and  anding  on  tha  laat  paga  with  a  printad 
or  illuatratad  impraaaion. 


Laa  axamplairas  originaux  dont  la  couvartura  tn 
papiar  ast  imprim*a  sont  filmte  an  commancant 
par  la  pramiar  plat  at  an  tarminant  soit  par  la 
darni*ra  paga  qui  comporta  una  amprainta 
d'impraasion  ou  d'illustration,  soit  par  la  sacond 
plat,  salon  la  cas.  Tous  las  autras  axamplairas 
originaux  sont  film*s  an  commandant  par  la 
prami*ra  paga  qui  eomporta  una  amprainta 
d'impraasion  ou  d'illustration  at  an  tarminant  par 
la  darniira  paga  qui  eomporta  una  talle 
amprainta. 


Tha  laat  racordad  frama  on  aach  microficha 
shall  contain  tha  symbol  —^  (moaning  "CON- 
TINUED"), or  tha  symbol  V  (moaning  "END"), 
whiehavar  applias. 


Un  das  symbolas  suivants  spparattra  sur  la 
darni*ra  imaga  da  chaqua  microficha.  salon  la 
cas:  la  symbols  — »■  signifia  "A  SUIVRE".  la 
symbols  ▼  signifia  "FIN". 


Mapa.  platas,  charts,  ate,  may  ba  filmad  at 
diffarant  reduction  ratios.  Thosa  too  larga  to  ba 
antiraly  included  in  ona  axposura  ara  filmad 
baginning  in  tha  uppar  laft  hand  eornar,  laft  to 
right  and  top  to  bonom.  as  many  frames  as 
required.  The  following  diegrams  illustrate  the 
method: 


Lea  cartes,  plonches,  tableaux,  etc..  peuvent  *tra 
filmis  i  des  taux  da  reduction  differents. 
Lorsque  le  document  est  trop  grand  pour  *trs 
raproduit  en  un  seul  cliche,  il  est  film*  *  partir 
da  Tangle  sup*rieur  gauche,  de  gauche  A  droits. 
et  de  haut  en  bes,  en  prenant  le  nombre 
d'imeges  n*cessaire.  Lea  diegrammas  suivsnts 
illustrent  le  methode. 


1 

2 

3 

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2 

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(ANSI  and  ISO  TEST  CHART  No    2) 


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ST' J!  Roch«5tef.    Htm    York  T4b0i*        USA 

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SPECTROSCOPY 


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THK   PLANK  (;K.\ri\(;   FOR  STKl.I.AK  SPI.(  TKOSCOPV 

ii\  J  s  i'i.\ski  rr 

At  the  nu't'linnol  the  inn.niittif  of  thf  A-lroimntiial  ami  A>tr(i 
physical  Soiiety  of  Aiiu-rira  on  Co  o|'tr,ition  in  Kailial  \«'lix  ily 
Determinations  at  Mt.  Wilson  in  luio.  the  iiuf-tion  of  in<  reasinj; 
the  eftic'ienty  of  the  nuMJern  >tcllar  spec  tro^;rai)h  wa>  <li>ni-i  <l  ami 
various  siij;j;eslions  were  olTered.  Tlie  writer.  «illi  the  loioent  of 
Dr.  King,  tiireetor  of  the  Dominion  ()^)^er\.ltory.  (ieiiiied  to  te>t  a 
plane  j;ratin>;  as  the  dispersion  piece  of  a  >tellar  >]  e*  tro^'raph. 
Some  earlier  experi.iients  on  the  u>e  tif  );ratinj;s  in  stellar  spei  tro>- 
copy  were  mad  •  hy  I'oor  and  Mitihell.'  l>y  \Vad>worth;'  and  l>y 
others  hut  without  very  em oura^iti^;  re>ult>.  The  ditliiulty  was 
probably  due  to  the  loss  of  li>;hl  entailed  by  Its  distribution  over 
several  s|)ectra  and  ean  be  o\ere<>nie  by  obtaining  a  ;;r.itin^'  frivinj; 
a  strong  conientration  in  one  order.  (Iratings  have  been  ruled  by 
Rowland  giving  a  very  strong  tirst-order  s|)eetruni.  ami  both 
Michclson  an<l  Ames  considered  that  it  >hould  be  possible  to  rule 
gratings  giving  a  large  proportion,  as  nnieli  as  75  jier  lent  of  the 
incident  light,  into  «)ne  t)rder.  K\en  if  this  etfuitncy  was  not 
reached,  grating  sj)ectra  should  ioni|>are  fa\<trably  with  those  Dro- 
(luced  by  three-prism  dispersior  especially  toward  the  \iolet.  where 
the  dense  flint  glass  used  in  most  prism  trains  is  strongly  absorbing. 

Consequenth-  a  i)lane  grating  to  give  the  strongest  jxissiblc 
tirst-order  siH'ctrum  was  ordered  through  the  J.  A.  Hrashear  Co., 
in  Octolx'r  iqio.  was  ruled  by  Dr.  J.  A  Anderson  of  Johns  Hopkins 
University,  and  received  in  January  igi  j.  The  grating  is  a  s-in»h 
plane  with  ruled  surface  7.4XQ.4  im  (.jJX.^J  inches),  ruling  15.000 
to  the  inch.  The  total  numln'r  of  lines  is  5.5.875.  which  will  be  the 
resolving  power  in  the  first  order  if  the  whole  aperture  is  used.  It 
was  estimated  by  Dr.  Amlerson  to  dilTract  alnnit  50  per  cent  of  the 
incident  light  into  one  first  order,  but,  as  will  be  seen  later,  this 
seems  to  be  an  o\-er«;stimate.' 


'  Aslrnphysifal  Journal,  7,  15;.  iSo-**- 
'Ibid.,  7.  ii^S,  i,Si)8. 


1  See  note  at  end. 


MS 


A74 


J.  .V.  rr.ASKETT 


{'onsiiU'raldr  thought  wa>  ^'i\rn  to  the  optiial  ilc^inn  of  llu" 
sp<'(tro>;ra|>h  ami  three  nulhods  of  [irndiu  iiig  a  >piitriini  wtTf 
<oml»iiiiil  ill  the  one  iii>trunieiit.  in  tlie  l'ir>t  of  the>e,  the  usual 
method,  in  whiih  a  |iarailel  |>ei)(il  Iron)  a  doulilet  (ulliinator  ol>jei - 
tive  of  57  mm  (.'.JS  in.)  a|)erture  is  ilitTraded  liack.  makinj;  an 
annle  of  alwiut  ,^o  with  the  initial  direi  tion.  is  emi>loyed.  th'.'  spei- 
trum  luini;  formed  hy  a  triplet  i  aniera  (shjeitive  of  ri^  4  mm 
h  5  in.  I  aju'rlureand  507  mm  (join.)  foi  us.  In  the  second  methiKJ 
the  autoeolliiritiiij;  or  I.ittrow  prim  iple  is  umiI.  the  (onihined 
eollimator  and  (amera  ohjei  ti\e  heinfja  triplet  of  6,^.4  mm  ( .' .  5  in.) 
aperture  and  051  mm  (,,7  s  in  )  foeus.  I'he  sjie'trum  is  formed 
in  the  plane  of.  and  as  » lose  as  possible  to.  one  end  of  the  slit.  The 
same  prineiple  is  used  in  the  third  method,  the  gratinn  Ininn 
replaeecl  \>y  a  hahprism  of  o.  10.?  k'I-'ss  haxinj;  anjiles  of 
58?5,  and  yo°,  and  >ilvereil  on  the  side  opposite  lli'  58?5  ijjle. 
The  light  is  refracted  into  the  prism  on  the  hy|)otenu>e  an<i.  when 
at  mini.  ,um  deviation,  is  imident  normally  on  the  silvered  side 
and  returned  along  its  original  path,  e.xeept  for  the  slight  imlina- 
tion  netessary  to  hring  it  on  the  jilate.  i  he  u>e  of  the  half-prism 
for  radial  velocity  work  was  jjrojiosed  l»y  Profess'  r  Caniphell  at 
the  meeting  above  referred  to.  and  I  understand  a  stellar  siiectio- 
graph  has  been  eonstrueted  at  the  Liik  Observatory  from  his 
designs  whii  h  gives  very  i>romising  results.  Neverthele.ss.  as  no 
ihange  was  nee<led  in  the  grating  si)e< trogra])h  excej)t  the  substi- 
tution of  the  half-prism  for  the  grating,  it  was  thought  worth  while 
to  n..ike  tests  of  this  type. 

The  linear  di>])e'-sions  of  tlie  three  forms  are  ^vo  A.  17.5  A, 
and  17.5  A  per  millimeter  at  //,.  These  values  are  almost  identical 
with  those  given  by  >i)ectrograi)h  I  (,^3.4  A)  and  spectrograph 
III  S  (17.5  A),  one-  an<l  three-])risni  spectrographs  of  the  Dcmiin- 
ion  Observatory,  thus  enabling  accurate  comparisons  of  relative 
intensities  to  be  easily  made. 

The  mechanical  ilesign  of  the  .spectrograph  folli.w.s  the  sup- 
j)orled  bo.\  form,  first  introduced  by  Campbell  at  the  Liik  Observa- 
tory, of  which  successful  examples  besides  those  at  Mount  Hamilton 
and  in  Chile  are  the  Mellon  .spectrograph  at  Allegheny  and  the 
single-prism  sj)ectrograi)hs  at  Ottawa  and  Ann  Arbor.     'J'he  box  is 


////:  ri  iv/  (.A'  i/7V(;  lok  >////  ia-  s/Vi  i Ho-^i  nrv    ^75 


mailf  of  lira>-'  plait-.  tirniU  ^inwiil  tuyttlur  .il  llic  .m^'lc^  ainl 
tln>r<>u(ihl\  l)ran'il  iiitfriially.  ( >\vinu  In  il>  nlaliM-  i  Minpat  liu»> 
a^  < onipari'il  with  llic  ( )llawa  uiu- |>ri~iii.  it  \\a>  not  iImiiiiIiI  iit«i>- 
sarv  to  ii'.truilim-  a  lliinl  i  ouiiliilMlaiu  ini;  >tippoit  a^  in  llir  iailtr 
iii>tnmifnt.  l>ul  thr  l»ox  i>  lulil  tlc\il>i\  ami  \il  tirnil\  in  llii- 
hrainl  T  iron  ir.mic  l>\  two  support-  -o  platcil  a>  to  riilinf  llrMirc 
to  a  niininuini.  I  lie  ohjti  li\i-  wliiili  <ltlnif  iHatitiliillv  .  and  llic 
ri'tU'ttinf;  slit  wvrv  nia«lf  \>\  the  J.  A.  Hia~luar  Co  Iml  all  oilur 
parts  of  tlu-  in-trnimiil  were  i  nn-trui  tctl  in  ilu'  ol)-tr\alor\  work 
shop. 

I'rfliniiiiary    lots   a'nl    ailjn-lnniit-    ni.,.ic    in     the    ial>oralor\ 
usin^  till'  iarl)oii  an    a-  tlu    li^ht  -ouric.  sliow<i|    that   the  luM  of 
thi'  s|K'i  trnni  \\.i>  pr.ntiially  ilal  o\it  the  4  (iti;rti  -  iiu  hultil  on  the 
plate  and  that  tin-  lU  imition  of  all  thrte  forms  ua-  1  vicllinl. 

I  111'  nii'thod  adoi)ti<l  for  ohtaininu  the  <oniparali\r  inlrn-itits 
of  prism. ilii  and  jrratin^  •luitra  was  to  makr  i\po-urt-  -nut  — 
si\t.'l_\  on  the  same  itlf-lial  ohjtt  Is.  both  sun  and  stars  l)iin^;  umiI. 
rive  spei  tra,  side  hy  -idt  on  the  s.ime  plate,  were  made  of  the  ohjei  t 
hy  eaih  instrunn  nt.  the  expti-u-e  time-  in  extrs  la-t  lieini;  l>ro- 
portional  to  the  '  ndxrs  1 .  1.5..'.  ^.4.  Inr  ixanipie,  on  /Vixyc;/, 
5  1  xpo-ures  wi  made  with  III  S.  ^  with  the  I.ittrow  irratin^;  ^ 
with  the  I.ittn  .lalf  |)ri-ni.  and  then  ;;  with  III  S.  all  three  m-tru- 
ment-  having;  the  -ame  dispersion.  .\  similar  le-t  wa-  arried 
throuf;h  with  the  lirst  foriii  of  the  ,L'ratin>:  spei  ito>;rai)h  and  the 
one-prism  instrument,  both  of  the  same  di-|itrsion.  ami  reiieated 
!  1!  ther  stars  and  lor  the  -un.  .\s  the  linear  disper-ions  were  the 
sanie  the  spettra  of  the  same  width,  the  tihservin^  tonditions 
f   irl\     ■instant,  tlu   ■  \po-ure  titiies  the  -ame.  •  nd  the  plates  dexel- 

I   tomther   for   tli.    same   lime,   the  intensity   of    the   re-ulting 

e-  v;i\es  a  direi  I  1  om|>arison  of  the  intensitx  ol   the  spet  tra. 

'\   equal  iniensities  on  the  plates  tan    he  tomjiareii.  liltk 

■)hi(    errc     -  likely  to  otiur.      '!  he  re-ull-  of  these  toin- 

re  ^i\eii  •!!  the  -uhjoined  tahle 

1  -I  striUns,'  feature  in  the  aiii>earann'  of  the  ;;ratin!;  as 

,..-np..i:  h  tlu- pr  -m.ru  speitr.i     and  this  is  al-o  e\  ideiit  Irom 

he  talii'  reni.irl-  d)le  uniformity  in  inten-it\    hetwieii  llg 

.iiid  /  ,;^  I  I'.i-  unifo'    iity  is  i)erhaps  the  mo-t  u-eful  property 


370 


./.  v  /•/.  i.v. 


„   ^  „.,,,.,   _ o   O    O 


I .  -,  ^  o  ?  J  ^  ^  -.  o  x.  .'  -^  o  «  -r  - 
•'  "  "  "  "  "  ' -.-..-_  o  o  o 


o  o  '^  "■  r,  \e  •*_  ^  t,  '^.  -m 
'■  t  ~r  '• ooo 


OOOO-OOOOJi-^-. 


■r  ^    ^    .'  .r   ^  .^  -  „    -. no/ 


■  o  o  o  r.  r.  -^  -r  o  <  c  t  -.  o  vc  t  •> 
.^  .■  .r  .,..,,,,  „ ooo 


o  o  -■  o  ? 


•  -  a:  -<-  - 
-ooo 


oouoooooojv:  c*^, 

"""------OOOO 


O    ",  .^    O    O    3   -«■  O    ",  -.    O    I  -  -^  O  •<  C    •• 
"""•--"■'""'"■■■' OOO 


OOO"'.  "-.  ' 


o  I  -  -r  o  :r  »--  - 
-• ooo 


o  o  "--  o  >: 


.J 

M 

a: 


CO-^--.  -.    -ooo- 0003CN 

° 000 

O    O    ■...'.  o    .<-.  ox   C    OXC    -'O   .--. 

•"^  -^  -^  -^  -  - o  o    : 


--.    >r.   A,    ,ri  Q    -^   o    -■'    1-^  *r,  '^ 

-r  -f  ./-,  -r  "-,'•-.-  o  o  o 


3  -^  »^'  "■,  -    O    O 


o  O  O  ^  X  »*: 
-  —  ooo 


•^t^^?5:  5^:;^:;  ^%=-x  1,^.1.- 


T  'K  ri.ASI   CUM  ISC  FOK  KIF.U.AR  SfKl  IKiiSu  >"]      ,77 

cif  the  Kralini;  |ieitrof;ra|>l).  '\hv  mrtrast  lutwcen  Kratin*:  in.l 
pri^m  in  this  ri'.|Hi  t  i>  v«ry  striking;,  as  pri^n.atii  -\>v,  tra  an-  hmt 
ten  tinifs  (.'rating  -h-i Ira  only  .nif  an'i  a  thhil  tini;>.  a-<  intfiM  ,it 
A  4700  as  at  X  ,<()00.  Thi-.  «.  TiTituc  i>  Im-  in  the  nviii  to  two 
causfs:  lirst,  the  im  reast-d  tlisiR-rsion  towanl  thr  vii.lti  .nil  liimin- 
ishfil  toward  the  red  of  pri>niatii  >|ri  tra  (dis|Hr.ion  at  K  •  nc  and 
a  half  tinus.  at  Hg  two  thirds,  that  at  //,>.  whiliMiilTrat  ti.in  -.jK-itra 
are  nearly  normal;  ami  semml.  the  strong  absorption  oi  the  pri>in 
glass  for  the  shorter  wave  lengths.  Caliulation-  from  Vo^eis  am- 
stants  for  o  loj  ^lass  sho».  that  {hrou;;h  5  mi  01  thi-  i;las»,  alwiut 
the  mean  lenjjth  of  path  through  the  >.ini;le  and  lh<'  hall  pri>m  \\ 
|)er  tent  is  transmitted  at  the  K  line  as  tompared  with  71  per  lenl 
at  //^  and  S^  per  »ent  at  //».  The  elTei  t  of  thi>  alivirption  is 
strikingly  shown  in  the  table  where  the  relative  inUn>ilv  u'  three 
prism  as  compared  with  sinjjie-  and  half  pri>m  ?peitra  toward  the 
violet  is  gi\en. 

Disnissinn  lirst  the  relative  inter. Nity  of  thrte  pti  m  and  uraliiiK' 
soettra.  we  find  the  former  has  the  advanta  ;e  from  lla  to  aliout 
A4<cx3.  while  from  /4.'oo  ilown  >;ratinn  s|H'etra  .ire  deiidediv 
iperior.  t  •  ,)rism  speitra  (.is;.  arinj;  helow //j.  If.ispettrum 
.ii  which  tn.  region  from  A  4^00  to  the  ultra  violet  1-  rei|iiiretl  onlv 
the  grating  could  be  used,  while  from  //«  to  //,  the  |)risms  would 
have  the  advantage.  In  other  words  for  early  t\pe  >t.ir>  u-i  the 
grating,  for  solar  t>-j)e.  three-prism  dispersion. 

Comparing  next  grating  with  single  and  half  prism  spectra  we 
tind  the  advantage  lies  decidedly  with  the  latter  ahove  the  H  am! 
K  lines,  but  if  the  K  line  is  required,  as  is  the  ca>e  in  many  c  irly 
type  stars,  it  can  be  obtained  with  the  gratin'i  in  the  .ime  time  as 
with  the  one-prism  si)ectrograph  with(>ut  making  the  lines  l;et"iin 
Ha  and  I/y  immeasurable  by  overexiM)sure. 

If  now  the  jmsmatic  spectra  are  intercon*.  •"•■d  the  superiority 
of  one-  over  three-prism  sjiectra  i>  markedly  show  •,  The  former 
gives  nearly  three  times  the  ini  isity  above  /  4.'oo  and  a  nun  h 
greater  ratio  below,  a  difference  of  mo-c  than  a  m:'gnltutle  in  the 
stars  within  reach  of  the  telesco[)e.  It  must  n.it  Ik  forgotten, 
however,  that  this  dilTerence  is  partially  olTse«  'v  the  threefold 
greater  resolving  j)ower  of  three  prisms,  although  -.a  i)hotographit 


.,.„.,       Th.  c..n„an,.  .ktvr.nin.n,   .he  roMt,..,.  of  ,lu  ,..„.,,.. 

i„,„ps  M..UUnh   to  :•..•,■.'.-.-  when  the  rUw-ly  .lonhk.l  ...,^s  ap,.ar. 
i..a\cn,an,s,,acticalU  o...,antM.h,n«a.  th.  ah..r,u....u. 

is  present  ( )r.  Jnne  1"  no  l.s.than  n-nel.en  ...n^.-.nt.v.  //  n,  s. 
be.,nn,n.  at  //..  ^^eK•  n.eaM.r..l  an.l  thu.  n.u.c  u.  r.  s  ..hU- 
Th.  -.vera^c  .kv.at.on  of  the  ocn,,.nte.l  ,...Mt,ous,  Haln.er  .  on- 
Htant  ■;(•.' 1  '.'■•  -t  the  niuWlc  th.rtc.n  of  thc^c  lines  fum  ih.  ob 
:.,,v.t  Jas  ..nW  ..O:  A.,  ana  of  nine  onlv  n04  A  ,  .how.n,  that 
U,.v  are  nnaonhtc-aiv  a.te  to  lnaH.,.n.  d.splacea  hv  .,on>e  nn 
,  ,u,wn  ana  iaent.oal  cause  f.on,  thcr  normal  ,>osU.ons. 

The  a,M.IaocnK-nt    .,f    nu.nv    euhancea    n-.etall:c   hnes  uh.ch 
wasd,scover.ai.v  Messrs.    Aaan.s  atui   Jov,   .,f    Mt.    Wilson   a.>a 
,,„.onncea   as   •-':;  A.  at    U^^m   has   been    ftdly    contunKa   ,n  the 
,,,ecira  obtains!  here  between  June  10  and    June    1...       I  he  pos,- 
„ons  of  sotne  .„-,  nulalUc  line,  iaentthea  .hKliv  a.  aue  to    /,.  /v, 
(V     S-,     S     /-Have   bten    n.easurea    on    these   plates,  ana    U   h:.s 
Ueeti  show',,  b.-vona  aoubt  that   the   a.splacen.ents   are   .Iso   p,  ■ 
,,ort,onal  to  the   wave  lcn,th,  a,e  exactlv   the  same  as  those  o, 
Ihe  hv.i.   .,en  absorpt.on  ana  merea^e  fron,  spect.un,  to  spectrum 
n.exa.th   the  same   way.      These  a.spl  rcen.ents  m.rst   evu  entlv 
,11  he  ane  to  the  san.e  cause  but  what  the  can^e  .s  cannot  be  ae- 
,,„„,nea.      Ahhon.h  the  a.splacetnen.s    follow    the    -"-'=»-- 
,,U,evwereveloc,tvsh.lts,  it   seetns   to   me   very    unhkelv    that 
M.ch  enonnon.  vclocit.es  as  1  inO  kn.    per  ^cc    on  Jnne  10  tncreas- 
„„.  U,  lT:.Oon  June  :;4  conUl   he    present.      A    tu.ther  evulence 
•  ha  k  on  not  be  due  to  velocUy  is  guen   bv   the  secona   nanow 
d,~orKion  .erus  of  seven  Inu.,  also  nnnustakabh    ane  to  h>..ro 
„,„    who^e  shuts  wonia  conespo.ul  to  a  vcUhUv  ot   -:;<()  k,u    rn 
1„„L  i:,      tb.t  .^  to  sav  the  absotb.ns;  hyaiov:...   wonKl  have  two 
a.lTerentvelocme^  lOT'^   uui  :i::.  ..  kn..  per  -tc  at  the  same  tnue. 
When  these  a.sp'.  .cements  .o  evactl «   follow  the   Hahner  law 
,or  hvarouen,  and  an    of  the  same  amount  for  the  enhanced    n.s 
an.l  also  tor  cakinn,  as  wdl  be  seen  below,  it  seems  more  hkely 


tliat  till  \  iif  dm-  to  Mitiif  i>!i\*km1  i  aii-i-  .iitii'n  iii  t'.u-  «..irt:«' 
niaiitifi  ')ii  Itif  iiiohviiU-"  .it  .ill  th«»«r  -.Iftnnit-  wli.H  the  utMii 
is  cat!  onh  W  mwe-iilatfl  on       tli.iii  to  vrloiHv 

rirli.it'*  tlir  iiio»l  r.ni.irkaltlf  1k.1i.m  ■!  ot  tlu  .^asrs  i-.  »lio«ii 
bv  thf  //aii'l  A'liitt"^  ot  .••ti-uim  winch  t.ikf  thu-r  tot  in* 

I  !  I  IvMw^ion,  not  viiv  »tton>;!v  ..uvti  I'tit  (m.|oiitii..llv 
ptrsi-nl  ill  -oiiif  ^Kfctr.j  aii.l  of  miihI  ii  wi.lth  .iiul  t>"-itioii  to  tlic 
livilro>;en  eitii»suin 

( 'i  )  iMspUifd  i»h-.ori'rioii  liiu'-.  that  of  A'  -liowiiiv;  aUviV- 
when  the  (Iwphu-ed  hy<honfii  alisotj  tioti  show^  inul  to  cxaotlv 
thf  same  snioiiiit  .itul  tli-^t  ,.t  // mncT.illv  .-Ifath  st-parai.-.l  troiu 
llu-  //.  al>^  .r|>tioii,  l.v  whu-h  it    <litlit>   in  w  ivf  '.■  ii^th    I's  a\w\\\ 

■1  A 

(I'O  Verv  narrow  an. I  >harp  iuu-  //  iii.l  A' ii«- ii  Iv  m  tin  ir 
normal  p«'-ition  atitl  jirtsriit  in  nll■^t  ot  llu-  s,„h  tra,  iv.  n  w  li.  n 
tliL-  hvilro.;t;n  aiwl  •)tlur  c  iltiniii  hI'v.i  ptioti  .Ik!  not  sli<<w  TIu-nc 
lines  art-  vcrv  accntatflv  iiu-asniaMc  aii.l  art-  avpiif'ilh  lunstain 
in  position.  ThfV  show  -.nir  induations  ot  >  *inali  .m  lu- 
tlnii-i-  in  \ilocltv  lint  tins  is  so  close  to  \\\v  prol.al.l.-  etr..i  ot 
iiik-asiiieniciit  th.nt  it  van  not  Ix-  r»u  if>if'l  -'^  ''^  "'V  in<-.iti*  . -tal. 
lislu.l  '1"1h-  n  ■  an  valiif  of  tli--  veloiitv  ot>ti»iiU'il  from  tlu-se  // 
iiui  A'lim-s  .,,1  17  platts  n-lnc..!  to  thi  v„n  is  t;(  7  U  4  km. 
]>fr  SCO  Consulerin-  tli<-  v.  l.Hitv  a.  constant,  the  proh.ihli-  .iror 
of  a  plate  as  Iclrrininf.l  l>v  thv  tnea«iire  «'  tlu-sc  two  lines  i,  1  7 
km.  per  sec-  Tilt-  i-arK  platfs  i;avc  a  vrlocitv  ot  ahoiit  17  km  , 
which  incrcas»r.l  to -J.S  m  about  K'  .iivs,  (IroitH-.l  to  !'.•  ami  in 
creas«<l  again  to '-':<,  hnt  altlioi  h  the  lim-s  auicc  \<iv  will  .nut 
c.in  hf  accurately  measnre.l.  the  ditT.t.  :icc  is  not  sutticient  with 
low  <lisinis;on  todtlinittly  ii^livatc  variation  in  vr-locitv  ami  we 
niav  slate  that  thi.  .I'.sorl.uij;  calcium  i-  moving  towartis  the  sun 
at  about  -Jtl  km.  pc-r  sec 

It  seems  hanlly  re.isonnMe  to  bdieve   that   the  fiiiissuc  c  il 
cinm     vapor,    that    «ivui-    the    displace.!    liiu-s    whose    position 
chaiivics  ill  a  week  lr.)m  Jn  A    to  .leatlv  •-'.'.  A.  tu  the  VMkt  of  the 
n.nmal  positmn,  and  that  Riving  the  narrow  sharp  lines,  c.iiistaiit 
.ti  p  isiti.m  and  w'.th  a  velocity  towards  the  sun  ot  '2l>  km   per  sec. 


.; 


378 


./.  .V.  ri.xsKErr 


spectra  thi'ir  lii<;h  ri'solutioii  taniiot  he  effectively  employed  and  it 
is  (if  IK)  value  in  early  Ixjie  stars. 

A  final  interesting;  mniparison  is  that  between  the  one-prism 
and  till'  half  |)risni  siieitra.  ihe  former  jjives  spectra  about  20 
l)er(ent  nuiri'  intense  than  the  latter,  a  result  to  be  expected  when 
the  los>  at  the  silvered  retlec  tinj;  surface  is  taken  into  account  and 
when  the  larj^er  aperture  of  the  half-|)rism  (0^.4  mm  as  comi)ared 
with  51  mm)  and  the  (()nse(|uent  f^reater  length  of  o|)tical  path  are 
considered.  It  is  evident  that  more  intense  spectra  would  be 
secured  by  usinj;  the  regular  one-prism  instrument  rather  than  the 
half  |)rism,  making'  the  camera  the  same  length  as  collimator.  The 
further  advantage  of  narrower  lines  for  the  same  slit-width  would 
be  obtained  by  increasing  the  length  of  lollimalor.  camera  remain- 
ing the  sanu'.  The  advantages  of  the  half-prism  instrument  arc 
it>  nun  h  simpler  and  more  comjiact  and  self-contained  mechanical 
form,  avoiding  >()inc  of  the  tle.xure  and  temj)erature  dilViculties 
occurring  with  the  extended  one-prism  spec  trogniph. 

In  cone  lu^ion.  it  ma\'  be  said  thai  although  the  spectra  obtained 
from  the  grating  are  disajipointingly  weak  and  show  that  liie  pro- 
|)oriion  of  the  incident  light  diffracted  into  the  spectrum  used  is 
nearer  ^o  than  the  50  per  cent  estimated  by  Anderson"  or  the  75 
per  cent  considered  po>sible.  yet  even  under  this  handicap  it  can 
be  used  to  ad\antage  when  the  K  line  is  re(|uire(l  and  if  spectra  of 
iMiilorni  intensity  or  of  uniform  disi)ersion  are  needed.  It  would 
also  be  Useful  in  the  red  ('\\i\  where  |)rismatic  sjiectra  are  so  unduly 
compressed.  If  a  grating  u'iving  twice  the  intensity  could  be 
obtained  it  would  be  sii|)erior  even  to  single-])rism  dis])ersion  for 
n\ii>t  work. 

The  relati\e  lle.xure  of  prism  and  grating  spec  trogra])h  should 
aUo  be  considered.  It  is  well  known  that  a  small  change  in  the 
position  of  a  pri^m  when  at  minimum  deviation  does  not  displace 
the  sjiectrum.  while  the  angular  movement  of  a  diffracted  pencil 
will  be  twice  that  of  the  grating.     Tests  of  the  single-prism  spectro- 

'I  li.u<'  Ir.iriuil  siiin-  lln-  .iliu\f  wm-  wriUi-ii  thai  Dr.  .XiidiTsuiiV  r>tinialf  rt-- 
Icrri-d  n.il  in  tin-  iru  iilinl  linlil  IjiiI  In  llu-  luTn-nlaK'f  ritiiriii'd  \>y  llii-  sliui  uliim  niftal 
Mni.in'  .\>  till'  ritli'i  li\  ily  ol  >]ifi  Ilium  iiu'tal  i--  .ilmiU  (15  perifiit  llii-  IvMn'sliriiaU-s 
an  III  i;(Hi,|  .mrcciiKnt. 


THE  PLASE  GKATIXa  FOK  SIEIJ.AK  SI'ECl  KOSiOl'l 


graph  showed  very  small  tiexures  of  the  onier  of  :  or  ^  km  jier 
second.  A  similar  te>t  of  the  h.df-prjsm  sjh'c  trojjraph  showed 
Maximum  tlexure  of  about  7  km  and  of  the  ^ratinj;  sjiei  troj;rai)h  of 
al)oiit  20  km  per  second.  In  most  exposures  the  disjihu fnieiit 
caused  l)y  the  change  of  position  in  hour  anjile  will  not  nun  h 
exceed  one-tenth  of  these  amounts,  and  it  is  evident  that,  willi 
proper  distribution  of  the  lomparison  exposure^,  there  >hould  be 
no  appreciable  elTei  t  on  the  \elocity  mea>ure>.  Ilenie  the  (|ues 
tion  of  tlexure  need  introduce  no  serious  ditVuultie>  in  the  u>e  of  a 
gratinj;  spectrofirajjh. 

I  have  much  pleasure  in  acknnwle<l,i;in;;  the  interest  of  the 
Director  in  this  work  and  his  wii!:n,t,'ne»  to  supi>ly  the  apparatus 
re<iuired. 

DoMlMllV    Ol!--!  K\  vniRS 

( )T  r  \«  \ 

Jamiiiry  loi  i 


